1. Field of the Invention
This invention relates in general to transportation apparatus, such as escalators, and more specifically, to escalators having improved braking control which includes a reference speed signal.
2. Description of the Prior Art
Escalators are provided with an electrically released, mechanically applied brake capable of stopping an up or down traveling escalator with any load less than the design load of the brake.
In the down-operating mode, when a stop command is initiated, the escalator may begin to travel faster, if there is a sufficient load, unless the brake is energized simultaneously with the stop command. To avoid this situation, braking action should occur as soon as the power is removed from the escalator. With the escalator fully loaded in down operation and the brake applied continuously, it may take several inches of travel to bring the escalator to a complete stop. With no load in the down travel direction, and with the brake applied at the same instant as the stop signal, the escalator may stop very quickly. It would therefore be desirable to modulate the braking action so that the stopping distance is approximately the same for both the fully loaded and unloaded down traveling escalator. Likewise it would be desirable to obtain the same deceleration rate for all load conditions.
When the escalator is operating in the up mode, and the brake is energized when the stop is initiated, the escalator may stop within approximately 1.5 inches under any load condition from no load to full load. If the brake is not energized with the stop command, the escalator may stop after approximately 16 inches of travel for no load and about 4 inches for full load. If loaded, the escalator reverses unless the brake is applied. A flywheel may be used to extend escalator travel in the up direction to obtain a smoother deceleration.
U.S. patent application Ser. No. 605,041, filed Apr. 30, 1984, entitled "Conveyor Brake Control", discloses an arrangement for obtaining a more uniform deceleration rate for different travel directions and loads. The apparatus discloses in this co-pending patent application uses a feedback arrangement in which a reference speed pattern signal representing the desired speed of the escalator is compared with a signal representing the actual speed. The reference speed pattern signal includes a constant segment followed by a linearly declining ramp segment. A signal representing the difference can then be used to control a motor and/or a brake, as required to follow the speed pattern. In this co-pending patent application, the initial value of the speed pattern reference signal is a fixed value.
During testing and actual use of the brake control apparatus described in the co-pending patent application, the results were not as uniform as expected. The response time of the brake, the load on the escalator and the effect of the dynamic braking of the energized motor, which is lost going into a stop mode, all interact to provide a non-uniformity in the ability of the control to cause the actual speed to quickly track the speed pattern. It was also found that the initial deceleration may become quite large before the escalator speed is brought under the control of the declining ramp segment of the reference speed pattern signal.